"Long before it's in the papers"
January 27, 2015


“Trust hormone” may drive aggression between groups

June 15, 2010
Courtesy of the University of Amsteram
and World Science staff

A chem­i­cal some­times called the “trust hor­mone” or “bond­ing hor­mone” be­cause of its role in so­cial rela­t­ion­ships can al­so pro­mote a type of con­flict, new re­search sug­gests.

Sci­en­tists have found that the com­pound, called ox­y­to­cin and pro­duced in the brain, leads hu­mans to sac­ri­fice their in­ter­ests for their own group while act­ing against out­side groups per­ceived as threat­en­ing.

The find­ings were pub­lished June 11 in the re­search jour­nal Sci­ence.

The re­search­ers, Carsten de Dreu of the Uni­vers­ity of Am­ster­dam and col­leagues, said the find­ings offer a bi­o­log­i­cal ex­plana­t­ion for why con­flicts be­tween groups es­ca­late when oth­er groups are seen as threat­en­ing. When such threat is low, for ex­am­ple be­cause there are phys­i­cal bar­ri­ers be­tween the group ter­ri­to­ries, con­flict es­cala­t­ion is less like­ly.

De Dreu and col­leagues con­ducted the study aim­ing to learn why ox­y­to­cin would pro­mote al­tru­is­tic be­hav­ior. Where­as clas­sic eco­nom­ic the­o­ry has trou­ble ex­plain­ing al­tru­ism, an ev­o­lu­tion­ary per­spec­tive sug­gests al­tru­ism func­tions to strength­en one’s own group, from which the in­di­vid­ual ben­e­fits in the long run. 

Be­cause ag­gres­sion to­wards com­pet­ing groups may help one’s own group to be­come rel­a­tively stronger, ag­gres­sion is an in­di­rect form of al­tru­is­tic, loy­al be­hav­ior to­wards one’s own group, some bi­ol­o­gists the­o­rize.

Charles Dar­win noted that groups whose mem­bers are al­tru­is­tic to­wards their own are more likely to pros­per, to sur­vive, and spread. De Dreu’s team rea­soned that if this is true, mech­a­nisms should have evolved that sus­tain al­tru­ism to­wards the own group, and ag­gres­sion to­wards com­pet­ing oth­er groups. The new find­ings sup­port this per­spec­tive, they ar­gued.

De Dreu and col­leagues con­ducted three ex­pe­ri­ments in which male par­ti­ci­pants ad­min­is­tered them­selves ox­y­to­cin or an in­ac­tive sub­stance, with­out know­ing which was which. The par­ti­ci­pants were then as­signed to small teams and in­structed to play games us­ing small amounts of mon­ey that they were pro­vid­ed. 

The games in­volved mak­ing a num­ber of con­fi­den­tial de­ci­sions on how to al­lo­cate mon­ey to them­selves, their team, and com­pet­ing groups. The “re­sults showed that ox­y­to­cin drives a ‘tend and de­fend’ re­sponse in that it pro­moted in-group trust and coop­era­t­ion, and de­fen­sive, but not of­fen­sive, ag­gres­sion to­ward com­pet­ing out-groups,” De Dreu and col­leagues wrote. When a com­pet­ing group was­n’t con­sid­ered a threat, ox­y­to­cin only trig­gered al­tru­ism to­wards one’s own group, they added.

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A chemical sometimes called the “trust hormone” or “bonding hormone” because of its role in social relationships can also promote a type of conflict, new research suggests. Scientists have found that the compound, called oxytocin and produced in the brain, leads humans to sacrifice their interests for their own group while acting against outside groups perceived as threatening. The findings were published June 11 in the research journal Science. When the competing out-group wasn’t considered a threat, oxytocin only triggered altruistism towards one’s own group, said the researchers, Carsten de Dreu of the University of Amsterdam and colleagues. This finding provides a neurobiological explanation for the fact that conflicts between groups escalates when other groups are seen as threatening, the investigators said. When such threat is low, for example because there are physical barriers between the group territories, conflict escalation is less likely. De Dreu and colleagues conducted the study aiming to learn why oxytocin would promote altruistic behavior. Whereas classic economic theory has trouble explaining altruism, an evolutionary perspective suggests altruism functions to strengthen one’s own group, from which the individual benefits in the long run. Because aggression towards competing out-groups helps one’s own group to become relatively stronger, aggression is an indirect form of altruistic, loyal behavior towards one’s own group, some biologists theorize. Charles Darwin observed that groups whose members are altruistic towards the own group are more likely to prosper, to survive, and spread. De Dreu’s team reasoned that if this is true, neurobiological mechanisms should have evolved that sustain altruism towards the own group, and aggression towards competing other groups. The new findings support this perspective, they argued. De Dreu and colleagues conducted three experiments in which male participants administered themselves oxytocin or an inactive substance, without knowing which was which. The participants were then assigned to small teams and instructed to play games using small amounts of money that they were provided. The games involved making a number of confidential decisions on how to allocate money to themselves, their team, and competing groups. The “results showed that oxytocin drives a ‘tend and defend’ response in that it promoted in-group trust and cooperation, and defensive, but not offensive, aggression toward competing out-groups,” De Dreu and colleagues wrote.